1. Field of the Invention
The present invention relates to a receiver suitable for receiving measured values transmitted through a two-wire transmission line.
2. Description of the Prior Art
In order to transmit outputs from a differential pressure transmitter, an electromagnetic flowmeter or the like representing measured values to a remote location according to conventional industrial measurement techniques, a unique signal having a current of 4-20 mA is used. An analog signal having a current selected from this range to represent a measured value is transmitted on a two-wire transmission line and is received by a receiver. Such differential pressure transmitters, electromagnetic flowmeters and the like are normally arranged in a distributed manner to monitor industrial process states in a wide physical area. Maintenance personnal must travel extensively to maintain and inspect the distributed measuring instruments so as to perform adjustments and check the operating conditions thereof. In order to eliminate such time-consuming maintenance or the like, existing equipment is utilized to achieve remote control operation of the measuring instruments, as described in U.S. Pat. No. 4,520,488.
As shown therein, a communicator is bridged to a two-wire transmission line to transmit a digital signal. At the time the digital signal is received by a transmitter, the transmitter stops transmitting an analog measured value signal and sends a response signal to the communicator. A similar mode of operation is achieved for digital signal communication between the communicator and the transmitter. The receiver converts the analog signal to a digital form and sends on the measured value in the form of a digital signal, thereby repeating digital signal transmission.
Since when the transmitter communicates with the communicator by means of the digital signal, the transmitter stops transmitting the analog signal, i.e., stops transmitting the measured value, if the measured value concurrently changes, the changed measured value cannot be immediately transmitted to the receiver. Therefore, the receiver cannot initiate an immediate control operation according to the changed measured value. This impairs the ability of equipment to be controlled by the receiver to follow changes in the measured values.
Further, the measured value to be transmitted from the transmitter to the receiver is sent in the form of digital signal, and the digital measured value is added to a response signal sent from the transmitter in response to the command signal from the communicator, whereby a composite signal is actually sent. However, if a digital signal excluding the measured value signal in an identical format is sent through a common transmission line, the receiver connected thereto receives all signals. In this case, the receiver receives digital signals in addition to the digital measured value signal in a mixed manner, and the control state of the receiver is disturbed which can produce an error in the control operation.